1. Field of the Invention
The present invention relates to a hydrogen storage alloy and an alkaline secondary battery using the same.
2. Description of the Related Art
A hydrogen storage alloy is capable of storing hydrogen safely and easily and therefore gains public attention as an energy conversion material and an energy storage material.
As one application example, a nickel-hydrogen secondary battery using a hydrogen storage alloy for its negative electrode is in great demand as a consumer battery for its property such as high capacity and cleanliness. A negative electrode using a hydrogen storage alloy consisting of a rare earth-Mg-based alloy has recently been developed with a view to actualizing a higher capacity of the nickel-hydrogen secondary battery.
The rare earth-Mg-based alloy is alloy obtained by substituting Mg for a portion of a rare earth element of a conventional AB5-type rare earth-based alloy. Compared to the AB5-type rare earth-based alloy, the rare earth-Mg-based alloy is characterized by a capability to store a great quantity of hydrogen gas at around ambient temperature.
On the other hand, the rare earth-Mg-based alloy has the disadvantage of low corrosion resistance against alkaline electrolyte. As a consequence, the secondary battery using the rare earth-Mg-based alloy for its negative electrode also has the disadvantage of a short cycle life.
Japanese Patent Application Publication NO. 2005-290473 discloses that the reason of the low corrosion resistance of the rare earth-Mg-based alloy is that magnesium easily reacts with alkaline electrolyte and creates hydroxides. The publication also explains that if the percentage of La and Ce in the rare earth-Mg-based alloy is set equal to or less than prescribed percentage, this prevents the generation of hydroxides, namely corrosion reaction, and elongates the life duration of the secondary battery.
The rare earth-Mg-based alloy disclosed in the above publication is surely enhanced in alkali resistance. However, the alkaline secondary battery employing this alloy is liable to be increased in battery internal pressure when it is charged after being overdischarged, for example, after it has been used for a long term as a power source of a low-power appliance, such as a remote controller, a clock, and a flashlight. Such increase of internal pressure is not desirable for the cycle life of the battery.